FIG. 1 illustrates a prior art test system 100 for testing dies of a semiconductor wafer 130. A semiconductor wafer 130 comprising a plurality of dies is placed on a moveable chuck 134 inside a prober 132 (which is shown in FIG. 1 with cut away 136 showing an interior 132 of the prober 132). The chuck 134 is moved to align terminals 118 of the dies with probes 116 of a probe card assembly 114, and then the chuck 134 is moved towards the probes 116 until terminals 118 are pressed against and form electrical connections with the probes 116.
Cable 104, test head 106, electrical connectors 108, and probe card assembly 114 include electrical paths that form a plurality of communications channels (not shown) between the tester 102 and individual probes 116. Once probes 116 are in contact with and thus electrically connected to die terminals 118, a tester 102 generates test signals that are communicated through the aforementioned communications channels (not shown) to dies of the semiconductor wafer 130. Response data generated by the dies of the semiconductor wafer 130 is communicated through the communications channels to the tester 102, which can evaluate whether the dies of the wafer 130 function properly.
Due, among other reasons, to the accumulation of debris on the probes 116, the electrical resistance of the probes 116 (which may be termed the “contact resistance” of the probes 116) can increase over time. As the contact resistance of the probes 116 increases, so does the risk that good dies of wafer 130 fail the testing not because the dies are faulty but because the contact resistance of the probes 116 interferes with the passage of test signals to and response signals from the dies.
It has been known to electrically connect two of the probes 116 to one input and/or output terminal 118 and then drive from drivers (not shown) in the tester 102 a current onto one of the two communications channels (each comprising electrical paths through cable 104, test head 106, connectors 108, and probe card 114) that connect the tester 102 to the two probes 116 and to measure, also at the tester 102, the voltage drop between those two communications channels. As is known from Ohm's law, the voltage drop between the two communications channels is equal to the product of the current driven onto the one of the two communications channels (and returned by the other of the communications channels) and the resistance of the two communications channels, the two probes 116, and the terminal 118 against which the probes 116 are pressed. The resistance of the two communications channels, the two shorted probes 116, and the terminal 118 against which the two probes 116 are pressed is thus the quotient of the voltage drop between the two channels divided by the current driven onto the one of the two channels.
It has been known to periodically determine the resistance of the two communications channels, the two probes 116, and the terminal 118 in order to track variations in that resistance. The usefulness of such determinations is limited, however, for several reasons. For example, the contact resistance of the two probes 116 pressed against the terminal 118 is often but a small part of the determined resistance, which as discussed above, also includes, among others, the resistance of the two communications channels connected to the two probes 116. Indeed, cable 104 is often many feet long. As another example, it has been difficult to obtain precise measurements of current and voltage at the tester 102. As yet another example, input and/or output circuitry, buffering circuitry, electrostatic discharge circuitry, etc. associated with the terminal 118 can affect significantly the current or voltage put onto the communications channels, which can significantly distort determination of the resistance. In some embodiments, the present invention provides improved methods, techniques, and apparatuses for determining information relating to contact resistance of probes in test systems like the semiconductor wafer test system 100 of FIG. 1 or any test system in which probes make electrical connections with a device being tested.